The present invention relates to the field of measuring the volume of liquid in a tank and in particular, to making such measurement using ultrasound.
Many liquids, like gasoline, are stored in large underground tanks where the volume of the liquid in the tank cannot be observed directly. The oldest and perhaps the most common way of determining the volume of such liquid is to insert a calibrated rod into the tank and read the height of the liquid from the line formed on the rod by the liquid's surface.
This method, however, provides only coarse estimates of liquid volumes because there are inherent errors in the system. For example, the person using the rod for measurement might not insert the rod perfectly vertically into the tank or might read the calibrations incorrectly.
The lack of precision provided in this method of volume measurement is most limiting in attempts to detect the slow loss of gasoline due to leakage. Leakage of gasoline from underground tanks can cause serious environmental problems and result in devastating legal liabilities. If such leakage could be detected at an early stage, the tank could be repaired or replaced at a fraction of that environmental and economic cost.
The use of a calibrated rod to measure the level of gasoline also has the added disadvantage of requiring the attendant to leave the security of the service station building. This can be dangerous for the attendant and it leaves the building unattended and vulnerable to robbery or vandalism.
One development in the area of liquid volume measurements has been ultrasonic ranging systems. One such system uses ultrasonic transmitters positioned above the liquid to measure the distance of the surface of the liquid from the transducer. Examples of this type of system are shown in U.S. Pat. No. 4,221,004 issued to Combs et al. on Sept. 2, 1980 and U.S. Pat. No. 3,184,969 issued to Bolton on May 25, 1965. These ultrasonic ranging systems are generally more accurate than a calibrated rod and they do not require a service station attendant to leave the building.
Unfortunately, these types of ultrasonic ranging systems are still not accurate enough for many uses. Such systems, for example, cannot determine whether a drop in the volume of the liquid is from loss of the liquid or from contraction due to a drop in the liquid's temperature. Adding a temperature detector will provide only limited correction since the temperature of gasoline usually varies over its volume and since there is no fixed coefficient of thermal expansion for gasoline which is nonhomogeneous and whose relative concentration of components varies. The knowledge of the temperature at one location in the gasoline is not very helpful when temperature-compensating for volume measurements.
Another attempt to improve the accuracy of ultrasonic volume measuring systems is the addition of calibrators. In U.S. Pat. No. 4,210,969 issued to Massa on July 1, 1980, a small sound reflecting target located a fixed distance from the surface of the ultrasonic transducer is used to help to correct for variations in the velocity of sound in the medium. The system in Massa, however, only detects variations of sonic velocity in the air above the liquid, and not variations in the gasoline.
The system described in U.S. Pat. No. 3,394,589 issued to Tomioka on July 30, 1968 is even more elaborate as it senses the reflections of ultrasonic energy off several equally spaced reflectors to make distance measurements. Although the additional reflectors can more accurately compensate for changes in sonic velocity through air, they also fail to account for changes of gasoline temperature.
Furthermore, ultrasonic ranging systems whose transducers sit above the gasoline cannot detect the presence of water in the bottom of the tank. If there is water below gasoline in a tank and a station operator, measuring the height of the liquid, does not recognize the presence of the water, the water may be accidentally pumped into a customer's tank. If this happens, the station operator runs the risk not only of losing customers, but also of possible legal action.
Conventional ultrasonic tank gauging systems whose receivers and transmitters are positioned at the bottom of gasoline tanks fail to measure the amount of water in such tanks reliably. Examples of such systems are described in U.S. Pat. No. 3,693,445 issued to Johnson on Sept. 26, 1972; U.S. Pat. No. 3,985,030 issued to Charlton on Oct. 12, 1976; and U.S. Pat. No. 4,229,798 issued to Rosie et al., on Oct. 21, 1970.
When the interface between the water in the gasoline lies very close to the transmitter and the receiver, ultrasonic signals reflected off the gasoline-water interface interfere with the transmission of subsequent signals and the tank gauge does not operate correctly.
The conventional tank gauging systems which have transducers beneath the surface of the liquid do not correct for temperature changes of the liquid. Of the systems listed above, only Rosie et al. measures the temperature of the liquid. Rosie et al., however, uses only a temperature sensor located at one position of the liquid.
One object of the present invention, therefore, is to measure the amount of liquid in a tank with great accuracy.
Another object of the invention is to detect very small leaks in a tank and the theft of small volumes of liquid.
It is also an object of the present invention to detect the presence of water in a gasoline tank and to measure the height of such water accurately.
Another object of this invention is a device and method for accurately measuring the volume of gasoline in an underground tank while avoiding the necessity of a service station employee's having to leave the service station building.
Yet another object of the invention is to measure the amount of gasoline in a tank even when no service station attendant is on duty.
A further object of this invention is to facilitate automatic system auditing so an inventory can be readily taken and product deliveries recorded, even when the service station is unattended.
Yet another object of this invention is to allow gasoline in a tank to be monitored frequently with little error and to provide early warning alarms to a service station operator for any unusual conditions.
Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will be obvious from that description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the methods and apparatus particularly pointed out in the appended claims.